When a printing plate composed of a photosensitive resin is produced, the photosensitive resin composition selectively exposed is developed by an aqueous developing solution, and the photosensitive resin composition of the non-exposed portion is dispersed or dissolved in the developing solution, for example. Here, when development of the photosensitive resin composition is repeated using the same developing solution, the concentration of the photosensitive resin composition dispersing in the developing solution is increased. As a result, reduction of development performance, adherence of a floating object to a brush and the like may occur. The non-cured photosensitive resin composition dispersing in the developing solution may adhere onto the surface of the printing plate again to deteriorate quality of the printing plate. For that reason, it is necessary to discard the developing solution having an increased concentration of the photosensitive resin composition and to resume platemaking using a fresh developing solution. Frequently repeated discarding of the used developing solution and replenishment of a fresh developing solution, however, are not preferred from the viewpoint of load to an environment and manufacturing cost. Then, Patent Literature 1 discloses a method for recovering a flocculate of a photosensitive resin composition floating in a developing solution by a filter.